The present invention relates generally to gas sensing equipment and calibration methods associated therewith. More particularly, the present invention relates to digitally-implemented methods for identifying and preventing xe2x80x9cdrift,xe2x80x9d or unwanted and inaccurate changes in the readings, of the gas sensing equipment.
Gas sensors are commonly used to measure the presence of one or more elements or components in a gas mixture. For example, a gas sensor may be used to measure the concentration of one or more pollutants in ambient air in an indoor or outdoor environment. Other examples may include flue gas monitoring, combustion processes control, fire alarms, gas leak detectors, and other applications where gas sensing is required. Such sensors are often connected to process control and monitoring equipment, or they may be connected to alarms that are triggered when concentrations of a measured gas exceed a predetermined limit, such as a desired limit based on worked protection guidelines or applicable regulations, or a health-based limit that is derived from research in the field.
A carbon dioxide (CO2) monitor, which measures the concentration of CO2 in ambient air, is one such gas sensor. Typically, CO2 monitors continuously measure CO2 concentrations in the subject environment and are connected to a ventilation control system to ensure appropriate ventilation rates. The nature of many environments, such as offices, stores, and industrial environments, results in a cyclical CO2 level in the environment at various points in the day. For example, an office area may experience its highest CO2 levels in the middle of the afternoon, when most CO2 sources (i.e., the office workers) are in the office and have been present for a period of time. Such an environment may experience its lowest CO2 levels in the wee hours of the morning, after most or all of the workers have departed from the previous day and before most or all the workers have arrived for the day.
Moderate natural background levels of CO2 exist in most environments. Thus, even when most or all CO2 sources are not present, as when all workers have departed from an office, a background level of CO2 typically remains. Thereafter, the background level may be considered to be a baseline against which measurements of concentrations in ambient air may be made.
Such background levels also exist in many environments for components other than CO2. For example, very low levels of carbon monoxide (close to 0.1 ppm) exist in most environments under normal conditions, and carbon monoxide alarms thus must be able to distinguish a measured concentration from almost zero levels of carbon monoxide.
One drawback of the prior art gas sensors is that such sensors typically experience a phenomenon known as xe2x80x9cdrift.xe2x80x9d Drift is a slow change of the properties of the sensor components, resulting in a gradual reduction in the accuracy of the sensor. Drift may be caused by any number of external factors, such as gradual chemical changes, a build-up of foreign matter over time and aging of the sensor components. The inaccuracy of the sensor is normally associated with changes in two sensor calibration parameters: baseline (zero readings) and span (calibration point). For many types of sensors, only one type of inaccuracy may dominate. Thus, for example, for one type of sensor an inaccuracy may be caused by the sensor zero drift, while another type of sensor may be inaccurate because of span drift. For example, it is well known that non-dispersive infrared (NDIR) gas sensors often primarily exhibit zero drift, while photo-acoustic infrared gas sensors typically have span drift as a dominating factor of inaccuracy.
A typical method for reducing or eliminating drift in a gas sensor is to either construct it to have high accuracy or to manually recalibrate the sensor on a periodic basis. Gas sensor re-calibration normally requires the use of at least one calibration gas mixture. Either option leads to increased costs, and both methods can be unreliable due to equipment failure and/or human error. Moreover, many applications require unattended sensor operation for very long periods of time, and thus frequent re-calibration is not desirable.
One method of compensating for drift of a gas sensor is disclosed in U.S. Pat. No. 5,347,474, to Wong. In particular, columns 3-6 of Wong, incorporated herein by reference, disclose a method of calibrating a carbon monoxide sensor. However, we have determined that alternate methods are desirable and preferable.
Therefore, we have determined that it is desirable to provide a method of automatically compensating for drift in gas sensors.
It is therefore a feature and advantage of the present invention to provide a method of automatically compensating for baseline and/or span drift in gas sensors.
One embodiment of the present invention is a method that includes providing a processor, such as an embedded microprocessor, with gas concentration data relating to a first period of time. The method includes identifying a quiescent period, which a subset of the first period of time. The gas concentration data that relates to the first period of time includes quiescent period data relating to the quiescent period. The method also includes determining a first component concentration, as well as providing the processor with at least one additional component concentration. Each additional component concentration corresponds to a separate and distinct time period. The first component concentration and the additional component concentrations provide an initial concentration set. The method also includes selecting, by the processor, at least one valid concentration from the initial concentration set. The selected valid concentrations yield a valid concentration set. The method also includes providing the processor with a preset background value, calculating, by the processor, an estimated background value, and calculating, by the processor, a correction value. The correction value in one embodiment may equal the difference between the preset background value and the estimated background value. Alternately, the correction value may equal the ratio of the preset background value to the estimated background value.
The method also includes the step of detecting a measured component concentration and adjusting the measured component concentration, the adjustment corresponding to the correction value, to yield an adjusted component concentration.
Optionally, the quiescent period has duration, and the duration is equal to or greater than a minimum duration, and the estimated background value relates to the quiescent period data, such as an average of some or all of such data.
As an alternative option, the quiescent period has a duration, the duration is less than a minimum duration, and the first component concentration is omitted from the valid concentration set.
As an additional option, it may occur that the first component concentration does not correspond to a predetermined value, in which case the first component concentration may be omitted from the valid concentration set. In this option, it is a further option that the predetermined value may relate to one or more of the additional component concentrations.
As additional options, the estimated background concentration may correspond to the valid concentrations in the valid concentration set, such as an average of some or all of such concentrations. Further, the method may include the additional step of triggering an event or selecting a restriction if the correction value exceeds a trigger value.
In accordance with an additional embodiment of the present invention, a system for compensating for gas sensor drift includes a processor and a memory for storing data to be processed by the processor. The processor is programmed to perform steps including accepting gas concentration data relating to a first time period. The steps also include identifying a quiescent period, where the quiescent period is a subset of the first time period. The gas concentration data includes quiescent period data relating to the quiescent period. The steps also including determining a first component level, accepting, from data stored in the memory, at least one additional component level, where each additional component level relates to an additional time period. The steps also include providing an initial component set that includes at least one of the first component level and one or more of the additional component levels. The steps also include selecting at least one valid component level from the initial component set to yield a valid component set; accepting, from the data stored in the memory, a preset background level; calculating an estimated background level; and calculating a correction value. Optionally, the correction value equals the difference between the preset background level and the estimated background level. Alternatively, the correction value equals the ratio of the preset background level to the estimated background level.
Optionally, the first component level relates to the quiescent period data. As an additional option, the estimated background level corresponds to the valid component levels in the valid component set, such as an average of some or all of such values. The processor may also be programmed to perform the step of detecting a measured component concentration and adjusting the measured component concentration, the adjustment corresponding to the correction value, to yield an adjusted component concentration. Also optionally, the processor is programmed to perform the step of triggering an event if the correction value exceeds a trigger value.
In accordance with an alternate embodiment of present invention, a method for compensating for the drift of gas sensing equipment includes gathering gas concentration data relating to a plurality of time periods and identifying a plurality of quiescent periods. Each quiescent period is a subset of the one of the time periods. The gas concentration data relating to each time period includes quiescent period data relating to the corresponding quiescent period. The quiescent period data includes data corresponding to a background concentration. The method also includes collecting the background concentrations for each quiescent period in an initial concentration set, validating the background concentrations in the initial concentration set to yield valid concentrations in a valid concentration set, identifying a first component level, calculating a second component level corresponding to the valid concentrations in the valid concentration set, calculating a correction value corresponding to a function of the first component level and the second component level, detecting a measured component concentration, and adjusting the measured component concentration corresponding the correction value to yield an adjusted component concentration. Optionally, the function in the calculating step is a difference or a ratio.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract included below, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.